Download 2.1 Chemistry’s Building Block: The Atom

BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Vital Harvest:
Deriving Energy from Food
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.
7.1 Energizing ATP: Adding a Phosphate
Group to ADP
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Energizing ATP: Adding a Phosphate
Group to ADP
• The molecule adenosine triphosphate (ATP)
supplies the energy for most of the activities of
living things.
• For ATP to be produced, a third phosphate
group must be added to adenosine diphosphate
(ADP).
• This process requires energy.
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7.2 Electrons Fall Down the Energy Hill to
Drive the Uphill Production of ATP
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Electrons Fall Down the Energy Hill to
Drive the Uphill Production of ATP
• In animals, the energetic fall of electrons
derived from food powers the process by which
the third phosphate group is attached to ADP,
making it ATP.
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Storing and Releasing Energy
2. Energy from food is then
stored as a phosphate bond
in ATP.
3. Energy is then released when
the phosphate bond is
broken, and can be used to
fuel our everyday activities.
1. Energy from food is
required to push a third
phosphate group onto ADP.
energy
in
ATP
energy
out
energy hill
P +
ADP
P +
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ADP
Figure 7.1
Storing and Releasing Energy
• Electron transfer in the production of ATP
works through redox reactions, meaning
reactions in which one substance loses
electrons to another substance.
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Storing and Releasing Energy
• The substance that loses electrons in a redox
reaction is said to have been oxidized, while
the substance that gains electrons is said to
have been reduced.
• In biological reactions, the loss or gain of an
electron is typically accompanied by the loss or
gain of a proton
• Thus, biological oxidation is the loss of 1 or
more hydrogen atoms, and biological reduction
is the gain of 1 or more hydrogen atoms
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Remembering Oxidation vs. Reduction:
OIL RIG
•
•
•
•
•
•
Oxidation
Is
Loss (of hydrogen)
Reduction
Is
Gain (of hydrogen)
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Storing and Releasing Energy
• In energy transfer in living things, starting food
molecules are oxidized, and the downhill fall of
the energetic electrons they lose ultimately
powers the uphill process by which ATP is
produced.
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NAD
• Electrons are carried between one part of the
energy-harvesting process and another by
electron carriers, the most important of which
is nicotinamide adenine dinucleotide or NAD.
• In its “empty” state, this molecule exists as
NAD+.
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NAD
• Through a redox reaction, NAD+ picks up one
hydrogen atom and another single electron from
food, thus becoming NADH.
• It will retain this form until it drops off its
energetic electrons (and a proton) in a later
stage of the energy-harvesting process.
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The Electron Carrier NAD+
empty
loaded
empty
goes to pick
up more electrons
proton
(oxidized)
used in later
stage of respiration
(reduced)
used in later
stage of respiration
1. NAD+ within a cell,
along with two hydrogen
atoms that are part of the
food that is supplying
energy for the body.
2. NAD+ is reduced to NAD
by accepting an electron from
a hydrogen atom. It also picks
up another hydrogen atom to
become NADH.
3. NADH carries the electrons
to a later stage of respiration
then drops them off,
becoming oxidized to
its original form, NAD+.
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Figure 7.3
7.3 The Three Stages of Cellular
Respiration: Glycolysis, the Krebs Cycle,
and the Electron Transport Chain
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The Three Stages of Cellular
Respiration: Glycolysis, the Krebs
Cycle, and the Electron Transport
Chain
• In most organisms, the harvesting of energy
from food takes place in three principal stages.
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Cellular Respiration
•
•
Cellular respiration is the harvesting of energy
from food.
It has three stages:
1. Glycolysis
2. Krebs Cycle (also called the Citric Acid Cycle or
the TCA cycle)
3. Electron Transport Chain (ETC)
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Cellular Respiration
• Some organisms rely solely on glycolysis for
energy harvesting.
• For most organisms, however, glycolysis is a
primary process of energy extraction only in
certain situations, when quick bursts of energy
are required.
• But, it is a necessary first stage to the Krebs
cycle and the ETC.
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Cellular Respiration
• Glycolysis takes place in the cell’s cytosol,
while the Krebs cycle and the ETC take place
in cellular organelles, called mitochondria,
that lie within the cytosol.
• In prokaryotic cells, glycolysis and the Krebs
cycle both take place in the cytosol. ETC
reactions happen across the plasma membrane.
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Cellular Respiration
• Glycolysis yields two net molecules of ATP per
molecule of glucose, as does the Krebs cycle.
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Cellular Respiration
Suggested Media Enhancement:
Cellular Respiration
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Cellular Respiration
• The net yield in the ETC is a maximum of
about 32 ATP molecules per molecule of
glucose.
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Cellular Respiration
• Glycolysis and the Krebs cycle are critical in
that they yield electrons that are carried to the
ETC for the final high-yield stage of energy
harvesting.
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Energy Harvesting
(b) In schematic terms
reactants
glycolysis
(a) In metaphorical terms
2 ATP
insert 1 glucose
glycolysis
products
glucose
2 NADH
2 energy
tokens
2 energy
tokens
Krebs
cycle
cytosol
glucose derivatives
CO2
2 NADH
CO2
6 NADH
Krebs
cycle
2 ATP
2 FADH2
32 energy
tokens
electron
transport
chain
electron
transport
chain
O2
32 ATP
mitochondrion
H2O
36 ATP maximum per glucose molecule
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Figure 7.4
7.4 First Stage of Respiration: Glycolysis
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First Stage of Respiration: Glycolysis
• Glycolysis begins with a single molecule of
glucose. The ultimate products are two
molecules of NADH (which move to the ETC,
bearing their energetic electrons) and two
molecules of ATP (which are ready to be used).
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Glycolysis
• Glycolysis also produces two molecules of
pyruvic acid—the derivatives of the original
glucose molecule—which move on to the Krebs
cycle.
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Glycolysis
molecules in
molecules out
glycolysis
glucose
glucose
Krebs
cycle
electron
transport
system
glucose-6-phosphate
Red balls are
carbons and
gold ovals are
phosphate groups
fructose-6-phosphate
fructose-1,6-diphosphate
glyceraldehyde-3-phosphate
1,3-diphosphoglyceric acid
3-phosphoglyceric acid
pyruvic acid
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Figure 7.5
7.5 Second Stage of Respiration: The
Krebs Cycle
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Intermediate Step
• There is a transition step in respiration between
glycolysis and the Krebs cycle.
• In it, each pyruvic acid molecule that was
produced in glycolysis combines with
coenzyme A, thus forming acetyl coenzyme A
(acetyl CoA), which enters the Krebs cycle.
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Intermediate Step
•
There are also two other products of this
reaction:
1. One molecule of carbon dioxide, which diffuses
to the bloodstream.
2. One more molecule of NADH, which moves to
the ETC.
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Transition Between Glycolysis
and the Krebs Cycle
glycolysis
glucose derivatives
2 NADH
Krebs
cycle
electron
transport
chain
glycolysis
mitochondrion
pyruvic acid
cytosol
NAD+
NADH
coenzyme
A
to electron
transport
chain
CoA
acetyl coenzyme A
CO2
inner
compartment
Krebs
cycle
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Figure 7.7
The Krebs Cycle
• For each starting molecule of glucose, two
molecules of pyruvic acid go through this step.
• Thus, the step’s product per molecule of
glucose is two molecules of carbon dioxide,
two NADH, and two acetyl CoA.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
The Krebs Cycle
• In the Krebs (or citric acid) cycle, the
derivatives of the original glucose molecule are
oxidized.
• The result is that more energetic electrons are
transported by the electron carriers NADH and
to the ETC.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
The Krebs Cycle
• The net energy yield of the Krebs cycle per
molecule of glucose is:
– six molecules of NADH
– two molecules of FADH2
– two molecules of ATP
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The Krebs Cycle
glycolysis
acetyl coenzyme A
Krebs
cycle
oxaloacetic acid
electron
transport
chain
1.
citric acid
2.
6.
a-ketoglutaric acid
malic acid
3.
5.
succinic acid
4.
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a-ketoglutaric
acid derivative
Figure 7.8
7.6 Third Stage of Respiration: The
Electron Transport Chain
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Third Stage of Respiration: The
Electron Transport Chain
• The ETC is a series of molecules located within
the mitochondrial inner membrane.
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The Electron Transport Chain
• On reaching the ETC, the electron carriers
NADH and FADH2 are oxidized by molecules
in the chain.
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The Electron Transport Chain
• Each carrier in the chain is then reduced by
accepting electrons from the carrier that came
before it.
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The Electron Transport Chain
• The last electron acceptor in the ETC is
oxygen.
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The Electron Transport Chain
• The movement of electrons through the ETC
releases enough energy to power the movement
of hydrogen ions ( H+ ions) through the three
ETC protein complexes.
• They move from the mitochondrion’s inner
compartment to its outer compartment.
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The Electron Transport Chain
• The movement of these ions down their
concentration and charge gradients, back into
the inner compartment through an enzyme
called ATP synthase, drives the synthesis of
ATP from ADP and phosphate.
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The Electron Transport Chain
• In the inner compartment, oxygen accepts the
electrons from the ETC and hydrogen ions, thus
forming water.
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The Electron Transport Chain
glycolysis
Krebs
cycle
Mitochondrion
inner membrane
outer compartment
electron
transport
chain
H2O
O2
inner compartment
Electron transport chain
ATP synthesis
outer compartment
inner
membrane
NAD+
1
2 H+ + —
O2
2
H2O
ATP
synthase
inner compartment
ADP + P
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Figure 7.9
The Electron Transport Chain
• If oxygen is not present to accept the ETC
electrons, the entire energy-harvesting process
downstream from glycolysis comes to a halt.
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7.7 Other Foods, Other Respiratory
Pathways
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Other Foods, Other Respiratory
Pathways
• Different nutrients and their derivatives can be
channeled through different pathways in
cellular respiration in accordance with the
needs of an organism.
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Many Respiratory Pathways
• Proteins and lipids enter the metabolic pathway
for ATP production at different points than does
glucose.
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Many Respiratory Pathways
food
proteins
carbohydrates
fats
amino acids
sugars
glycerol fatty acids
glucose
glycolysis
pyruvic acid
acetyl CoA
Krebs
cycle
NH3
(ammonia)
electron
transport
chain
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Figure 7.10